Change from aerosol-driven to cloud-feedback-driven trend in short-wave radiative flux over the North Atlantic

Abstract

Aerosol radiative forcing and cloud-climate feedbacks each have a large effect on climate, mainly through modification of solar short-wave radiative fluxes. Here we determine what causes the long-term trends in the upwelling short-wave (SW) top-of-the-atmosphere (TOA) fluxes (FSWg↑) over the North Atlantic region. Coupled atmosphere-ocean simulations from the UK Earth System Model (UKESM1) and the Hadley Centre General Environment Model (HadGEM3-GC3.1) show a positive FSWg↑ trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We find that the 1850-1970 positive FSWg↑ trend is mainly driven by an increase in cloud droplet number concentration due to increases in aerosol, while the 1970-2014 trend is mainly driven by a decrease in cloud fraction, which we attribute mainly to cloud feedbacks caused by greenhouse gas-induced warming. In the 1850-1970 period, aerosol-induced cooling and greenhouse gas warming roughly counteract each other, so the temperature-driven cloud feedback effect on the FSWg↑ trend is weak (contributing to only 23 % of the ΔFSWg↑), and aerosol forcing is the dominant effect (77 % of ΔFSWg↑). However, in the 1970-2014 period the warming from greenhouse gases intensifies, and the cooling from aerosol radiative forcing reduces, resulting in a large overall warming and a reduction in FSWg↑ that is mainly driven by cloud feedbacks (87 % of ΔFSWg↑). The results suggest that it is difficult to use satellite observations in the post-1970 period to evaluate and constrain the magnitude of the aerosol-cloud interaction forcing but that cloud feedbacks might be evaluated. Comparisons with observations between 1985 and 2014 show that the simulated reduction in FSWg↑ and the increase in temperature are too strong. However, the temperature discrepancy can account for only part of the FSWg↑ discrepancy given the estimated model feedback strength (λ = ∂FSW/∂T). The remaining discrepancy suggests a model bias in either λ or in the strength of the aerosol forcing (aerosols are reducing during this time period) to explain the too-strong decrease in FSWg↑, with a λ bias being the most likely. Both of these biases would also tend to cause too-large an increase in temperature over the 1985-2014 period, which would be consistent with the sign of the model temperature bias reported here. Either of these model biases would have important implications for future climate projections using these models.